A conventional Time Division Multiplex (TDM) Passive Optical Network (PON) belongs to a binary digital modulation optical communication system. A Physical Medium Dependent (PMD) layer or Physical (PHY) layer of the conventional TDM-PON adopts a binary digital modulation technology. For example, a Gigabit Passive Optical Network (GPON) and a 10-Gigabit-Capable Passive Optical Network (XG-PON1) adopt a scrambled binary Non-Return-to-Zero (NRZ) code, and an Ethernet Passive Optical Network (EPON) and a 10G EPON adopt 8b/10b and 64b/66b line coding NRZ. A low-frequency component of an electric power spectral density of a binary NRZ code of the conventional TDM-PON is superimposed with an electric power spectral density of a Radio Frequency (RF) video signal transmission system.
When a Wavelength Division Multiplexing (WDM) technology is adopted for coexistence of an RF video system and a TDM-PON system in the same Optical Distribution Node (ODN), an RF video signal and a downlink signal of the TDM-PON system are transmitted in the same direction in the same optical fibre with their power spectral densities superimposed, which causes a Raman scattering effect of transmission of different wavelengths in the same optical fibre as well as nonlinear Raman crosstalk of the downlink signal of the TDM-PON system to the RF video system, so that quality of a part of a low-frequency channel of the RF video signal is seriously degraded, and the RF video system and the TDM-PON system cannot coexist in the same ODN; and moreover, the cost of an optical transceiver component of a 10 Gbps binary digital modulation optical transmission system has reached a bearing limit of a network access user, so it is urgent to look for a technical solution for lowering the cost.
A binary digital modulation mode is adopted in most of optical communication systems in a related art, greater bandwidths are required during transmission, and optical transceiver components with higher transmission rates are required for the greater bandwidths, so that higher using cost of users and poorer practicability are caused.